Inertial reference guidance apparatus, extensively utilized in aircraft and missile navigational systems, have traditionally employed spinning mass gyroscopes and associated electromechanical devices for performing various guidance functions, including the detection and measurement of angular rotation rates. Such devices are relatively bulky, expensive and complex, are subject to drift rates difficult to control, and require an extensive number of moving parts, some of which have a corresponding short operating life.
Various apparatus utilizing more sophisticated concepts than those of the spinning mass gyroscopes to detect and measure rotation relative to a reference frame have long been known in the arts of electromagnetics and, more particularly, optical physics. One of these concepts is the "Sagnac effect" manifested in implementations of ring interferometric apparatus and first demonstrated in now classic experiments described by Sagnac in 1913 and later by Michalson and Gale in 1925.
Ring interferometers comprise an optical or other electromagnetic wave source for generating a signal which is applied to a beam splitter or similar optical isolation device to divide the generated signal into two equivalent counter-propagating waves initially transmitted on separate paths. These paths respectively terminate at each of two ports of a closed ring configuration such that the paths are of substantially equivalent length.
The Sagnac effect can be simply described and understood by characterizing the counter-propagating waves as a clockwise (CW) wave and a counterclockwise (CCW) wave. If the ring configuration is rotating at a clockwise rate .OMEGA..sub.I, relativistic theory explains that the counter-propagating wave travelling in the same direction as the rate of rotation of the closed path (the CW wave) is observed to follow a longer optical path than the CCW wave travelling in opposition to the path rotation. The counter-propagating waves will therefore experience a differential phase shift P.sub.S (known as the "Sagnac effect" phase shift) which can be characterized in accordance with the following equation: EQU P.sub.S =[(4.pi.RL)/(W.sub.0 C)].OMEGA..sub.I (Eq. 1)
where R is the radius of the enclosed path, L is the actual length of the physical path, W.sub.0 is the nominal wavelength of the counter-propagating waves, and C is the speed of light.
As apparent from Equation 1, this Sagnac phase shift P.sub.S is linearly proportional to the angular rotation rate .OMEGA..sub.I of the passive ring for constant wavelength optical signals. Accordingly, a system having a ring interferometer and means for detecting and measuring Sagnac phase shift is theoretically capable of use as a navigation apparatus to determine angular rotation rates. However, as described below, extensive difficulties exist in developing physically realizable interferometric apparatus suitable for implementation in aircraft and, more specifically, in developing systems capable of practically and accurately measuring Sagnac phase shifts.
The early development of practical navigation apparatus employing Sagnac interferometric principles was hindered by the bulky size of requisite instrumentation components and direct measurement difficulties due to the small magnitude of induced Sagnac phase shifts in the range of rotation rates achieved during flight. However, laser technology and the recent advances in development of low scatter mirrors and stable structural materials have rendered the Sagnac effect measurable in various prior art systems. Certain of these systems, such as those disclosed in the Podgorski U.S. Pat. No. 3,390,606 patent issued July 2, 1968, utilize "active" medium ring configurations and are commonly known as "ring laser gyroscopes". These ring laser gyroscopes comprise tuned resonant cavities wherein the angular rotation rate of the ring configuration is proportional to observed beat frequency between the oppositely travelling waves within the cavity. However, such active medium ring lasers have problems associated with the phenomena of "pulling" "frequency lock-in" commonly known to those skilled in the art of optical system design. These phenomena are experienced when the frequency difference between the oscillating waves becomes small, for example, less than 500 Hz. Optical coupling occurring within the active medium tends to "pull" the frequencies of the oscillatory waves together (mode pulling) and ultimately "locks" them together (frequency lock-in) into one frequency, thereby eliminating beat frequency at the low frequency differences which would be observed in ring laser gyroscopes operating in aircraft or missile navigation systems.
Rate sensing devices have also been developed utilizing "passive" ring configurations wherein the ring configuration is a tuned cavity arrangement with externally generated counter-propagating waves. As the ring configuration is angularly rotated, the counter-propagating waves exhibit differential frequencies and, like the ring laser systems, a corresponding beat frequency is observed therebetween which is proportional to the rate of rotation. Though these passive systems do not experience frequency lock-in and pulling phenomena, other bias variation effects such as high temperature sensitivity will produce inherent beat frequency instabilities when the tuned cavity ring configurations comprise adjustable mirrors or similar arrangements. If optical fibers are utilized in the ring configurations, as may be necessitated to minimize instabilities, cavity length control becomes extremely difficult.
Another problem associated with any optical system employing signals having differential frequencies is that various bias effects can operate in a nonreciprocal manner dependent on wave frequencies. Such bias effects are cumulative over time and can result in observed finite beat frequencies even though there is no actual angular rotation of the ring configuration.
The state of the art of integrated circuit optics and, more specifically, optical fiber and laser design is now at a stage whereby compact instrumentation comprising passive ring interferometers can be designed with coiled multiple turn fiber optic rings capable of producing a measurable Sagnac effect phase shift over a substantially wide range of rotation rates as required in aircraft and missile applications. It should be apparent from Equation 1 that increasing the number of ring turns correspondingly increases the magnitude of Sagnac phase shift for a given rotation rate. These passive ring interferometers utilize single mode counter-propagating waves and avoid the problems of active medium and dual mode systems as previously described. However, existing rate sensing devices utilizing the aforementioned state of the art optical technology still exhibit inaccuracies caused by inherent problems such as poor resolution over wide dynamic ranges of rotation rates (e.g. low signal to noise ratios) and sensitivity to intensity and wavelength variations of source generated signals.
To illustrate certain of the aforementioned problems and for purposes of understanding the invention, FIG. 1 depicts, in block diagram form, a prior art rate sensor 100 having a passive ring Sagnac interferometer 101. The subsequent discussion herein regarding the Sagnac effect will be somewhat cursory in that detailed principles of such interferometers are well-known in the art and, for example, are described in Schneider et al, Journal of Applied Optics, Volume 17, page 3035 et seq. (1978).
Interferometer 101 comprises a laser source 102 capable of generating an optical signal on conductor 104 having a nominal wavelength W.sub.0. Conductor 104 and other conductors described herein can comprise any type of path capable of transmitting optical signals. The optical signal on conductor 104 is applied to a beam split/recombine circuit 106 as shown in FIG. 1. The split/recombine circuit 106 is an optical isolation/coupler circuit well known in the art of optical circuit design and divides the optical signal on conductor 104 into two equivalent counter-propagating optical signal waves transmitted on conductors 108 and 110. These signal waves will be referred to as the clockwise signal (CW) wave 112 as transmitted on conductor 108 and the counterclockwise (CCW) signal wave 114 as transmitted on conductor 110.
The waves 112 and 114 are applied respectively to the two ring ports 116 and 118 of a multiple turn fiber optic passive ring 120. Included in the path of conductor 110 is a phase bias circuit 122 which will be described in subsequent paragraphs herein. The fiber optic ring 120 is coiled such that it comprises a radius R and a path length L. The CW wave 112 and CCW wave 114 traverse the passive ring 120 in opposite directions and emerge from the ring on conductors 110 and 108, respectively. The returning counter-propagating waves are then again applied through circuit 106 and recombined such that a combined signal wave referred to as CS wave 124 is transmitted on conductor 126 as shown in FIG. 1.
The returning CW wave 112 and CCW wave 114 will experience a relative Sagnac phase shift having a magnitude and directional sense linearly proportional to the angular rotation rate of the passive ring 120. If the phase shift is characterized as P.sub.S and the angular rotation of the passive ring as .OMEGA..sub.I, then Equation 1 defines the proportional relationship.
Ignoring the function of the depicted phase bias circuit 122 and any constant predictable phase shifts within the interferometer 101, the recombined CS wave 124 will be reflective of the Sagnac effect phase shift P.sub.S and can be applied on conductor 126 to a photodiode 128. CS wave 124 will "impinge" on the photodiode 128 with a fringe pattern well-known in the art of optical physics. The "low order" fringe pattern, that is, the areas between alternate light and dark bands near the center of the fringe pattern, will vary in intensity in accordance with the relative phase of the recombined counter propagating waves 112 and 114 as represented by CS wave 124. The current output signal of the photodiode 128 on conductor 130 is representative of the intensity of the "zero order" portion of the low order fringe pattern. For purposes of description, this intensity signal will be referred to as signal S and can be applied as shown in FIG. 1 to various readout circuits 132 which provide a measurable output signal on conductor 134 corresponding to the signal S.
As known in physical optics theory, the signal S on conductor 130 can be described in terms of the following equation: EQU S=I.sub.0 cos.sup.2 1/2(P.sub.S +.pi.) (Eq. 2)
where I.sub.0 is the maximum signal intensity and P.sub.S the relativistic phase shift occurring due to the Sagnac effect as previously described with respect to Equation 1. FIG. 2 depicts the sinusoidal variation of signal S relative to the Sagnac phase shift P.sub.S. S is symmetrical about the axis origins with the intensity having a zero value for a zero value of P.sub.S. As shown in FIG. 2, if the intensity of signal S is measured as a value S.sub.1, then a corresponding magnitude of phase shift P.sub.S can be determined as phase shift P.sub.1. As previously described with respect to Equation 1, P.sub.S is linearly proportional to the angular rotation rate for a specific passive ring configuration and a constant wave length signal source. Accordingly, the magnitude of signal S provides an observable determination of rotation rate .OMEGA..sub.I. Other conventional circuitry, which need not be described herein, can be utilized to provide indication as to the polarity, i.e., directional sense, of the phase shift and to further determine whether the phase shift is between 0.degree. and 90.degree. or 90.degree. and 180.degree., etc.
The read out circuits 132 comprise conventional means of obtaining a measurement of the intensity of signal S. For example, signal S can be sampled with associated analog to digital (A/D) conversion periodically every T seconds. The resulting output of such digital mechanization can be a binary word proportionally representative of the angular rotation rate .OMEGA..sub.I each period. The period T must be chosen sufficiently small to preclude loss of signal information when computing the angular displacement from the samples of intensity signal S.
As previously noted, several problems exist in basic implementations of rate sensors employing passive ring Sagnac interferometers as depicted in FIG. 1 when utilized in inertial reference systems. The relationship between the intensity signal S and the Sagnac effect phase shift P.sub.S is a nonlinear sinusoidal cos.sup.2 waveform as described in Equation 2. The physically realizable values of P.sub.S will be extremely small with respect to the wave length W.sub.0. Accordingly, the actual measured intensity S.sub.1, corresponding to a Sagnac phase shift P.sub.1, will be close to the minimum "valley" of the wave form of the signal S. Therefore, measurement of changes in Sagnac phase shift by measuring changes in magnitude of signal S is extremely difficult. Thus, within this area of operation, the nonlinear relationship between the intensity of signal S and the Sagnac phase shift P.sub.S limits the useful range of rate measurements when utilizing conventional measurement techniques such as digital sampling. That is, any type of digital sampling to obtain an estimation of the Sagnac phase shift will be limited by the minimal sensitivity occurring at the valley of the waveform of signal S near the axis origins.
Another problem in prior art systems is related to possible intensity variations of the signal S. Such variations can readily occur due to laser source variations or transmission losses within the optical conductive paths of interferometer 101. FIG. 3 depicts the effect of signal intensity changes with the nominal waveform of signal S shown in dotted lines and the intensity varied signal S shown in solid lines. As apparent therefrom, an intensity change in signal S can result in an erroneous determination P.sub.E of the Sagnac phase shift P.sub.S for a measured signal magnitude S.sub.1. This erroneous determination will thus result in an erroneous calculation of the angular rotation .OMEGA..sub.I.
Another difficulty with interferometer 101 is the possibility of obtaining erroneous measured rates due to variations in wavelength of the optical signals. For example, a typical optical beam generated through a laser diode has a wavelength which is temperature dependent and may vary in the range of 0.03% per degree Centigrade. FIG. 4 depicts the effect of wavelength changes where the intensity pattern of signal S with a nominal wavelength W.sub.0 is shown in dotted lines and the varied pattern of signal S with an actual wavelength W.sub.E is shown in solid lines. Again, such wavelength changes result in an erroneous determination P.sub.E of the Sagnac phase shift P.sub.S for a measured signal magnitude S.sub.1.
Other problems also exist when utilizing interferometers in applications such as missile navigation systems where substantial accuracy is required over a wide dynamic range of rotation rates. For example, such a navigation system can require output signals indicative of rotation rate throughout a range of 1000.degree. per second to 1.degree. per hour, i.e., a range ratio of 3.6.times.10.sup.6 to 1 assuming constant resolution within the range. If a measurement technique such as digital sampling is utilized to estimate the magnitude of signal S, a 22 bit (plus sign) binary word must be utilized for purposes of analog to digital conversion. The necessity of such large scale data words is prohibitive to the use of small scale high speed A/D converters as required for aircraft and missile guidance control systems. Another problem associated with the requisite wide dynamic range pertains to the signal to noise (S/N) ratio. In accordance with conventional communication theory, a 131 db S/N ratio is required for a 3.6.times.10.sup.6 dynamic range. In physically realized passive ring interferometers comprising the circuitry shown in FIG. 1, the S/N ratio will actually be closer to a value of 75 db.
Certain prior art systems employing passive ring interferometers have attempted to overcome the previously discussed problem of intensity signal insensitivity to Sagnac phase shift changes by introduction of a phase bias circuit 122 into the optical conductive path 110 as shown in FIG. 1. Circuit 122 is a conventional circuit which induces a phase shift in wave signals transmitted on conductor 110. This externally applied phase shift modifies the previously described relationship of signal S to Sagnac phase shift disclosed in Equation 2 to the following: EQU S=I.sub.0 cos.sup.2 1/2(P.sub.B +P.sub.S +.pi.) (Eq. 3)
where P.sub.B is the externally induced phase shift applied from phase bias circuit 122.
The induced phase shift P.sub.B causes the relational pattern of output signal S to be "shifted" with respect to the Sagnac phase shift P.sub.S. FIG. 5 depicts in dotted lines the relationship between signal S and Sagnac phase shift P.sub.S with no externally induced phase shift and further depicts in solid lines the effect on the same relationship of the induced phase shift P.sub.B. As apparent from FIG. 5, the measured intensity S.sub.1 with induced phase shift P.sub.B and corresponding to a Sagnac phase shift P.sub.1 will be on a substantially linear and "maximum slope" portion of the relational pattern. In accordance with conventional digital sampling and communication theory, such a system will be substantially more sensitive to changes in Sagnac phase shift due to angular rotation rate changes than will a system where the expected values of phase shift occur on or near peaks and valleys of the sinusoidal intensity signal wave pattern.
One recently developed gyroscope apparatus utilizing a passive fiber ring interferometer and generally employing phase bias circuitry was invented by W. C. Goss and R. Goldstein and is described in the "Technical Support Package on Optical Gyroscope for NASA Technical Brief", Vol. 3, No. 2, Item 25, J. P. L. Invention Report 30-3873/NPQ-14258 published by Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. and dated October, 1978. The Goss et al optical gyroscope comprises a passive ring Sagnac interferometer for measuring rotation rates in accordance with the Sagnac phase shift principles previously discussed herein. Output signals are generated at two optical detectors having a response pattern indicative of the resultant phase shift due to angular rotation of the passive fiber ring.
A bias cell utilizing commonly known "Faraday effect" principle is introduced into the optical paths of the interferometer to provide a constant 45.degree. degree advance of one wave, 45.degree. retardation of the other wave, and phase offset compensatory for the Sagnac effect phase shift. The overall effect of the bias cell is to "shift" the response pattern of the output signals such that changes in signal intensity are maximized for corresponding Sagnac phase shift changes, thereby providing maximum measurement sensitivity. A fiber optic reversing switch is also included in the optical paths to minimize the phase shift effects of such reciprocal phenomena as long term source drift, etc.
The Goss apparatus is a substantial technological advance over other prior art rate sensing devices. However, it does not provide complete solution to inaccuracies in measuring rotation rates with passive ring Sagnac interferometers caused by inherent problems such as sensitivity to short term source intensity variations and optical path losses, wavelength dependency, lack of sufficient signal to noise ratio and insufficient operational dynamic range.